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Pixel and Microdot Detectors

Abstract

This chapter describes pixel, microdot, and micropixel detectors. Their invention was inspired by A. Oed's work on MSGCs. In these detectors avalanche multiplication occurs near small anode dots/pixels instead of near anode strips. This naturally segments the detector area into independent active cells, or pixels, which can be attractive in some applications. For two-dimensional position measurements, each anode and cathode row is connected to the readout line. These readout lines are placed perpendicular to the anode/cathode rows. If necessary, each pixel can be connected to its own preamplifier. One of the advantages with this pixel geometry is that it allows gas gains that are ten times higher than what is achievable with MSGCs. This is due to that the electric field lines near the anode dots are radial, which is favorable for quenching surface streamers. Although up to now the microdot and micropin detectors remain in a stage of laboratory prototypes, this interesting concept recently gained new momentum after the development of microdot detectors with resistive electrodes. These innovative detectors satisfy the requirements of some application such as noble liquid Time Projection Chambers.

1. Main Designs And Operating Principle

The pixel detector was one of the first new designs of micropattern gaseous detectors developed shortly after the invention of the MSGCs. The first prototype of this detector type was described by Mattern (1991).

The detector is made from kapton which is metallized on both sides. On one side the metal is etched into circles with a diameter of ~4 mm. In the center of each circle is left a small metalized anode dot with a diameter of a few hundred μm. The anode dots are fed with high voltage from the other side of the kapton through thin plated holes (see Figure 1).

Figure 1.

One of the first designs of a pixel detector operating in limited streamer mode (Mattern, 1991)

As in the case of the MSGCs and MGCs, a drift electrode is located a few cm above the 2D (two-dimensional) electrode structure. Primary electrons created by radiation in the drift region move towards the anode dots and produce avalanches there. Even at high gains the avalanches do not transit to sparks due to the coaxial geometry of the electric field along the dielectric surface. Any streamers produced will propagate some distance toward the cathode in the diverging field and then self-quench without reaching the cathode. This detector can be considered as a 2D version of an array of miniature single-wire counters described in a paper by Karabadjak (1983).

With microelectronic technology it is possible to produce very small cells. A later and improved version of the pixel detector is the microdot detector suggested by Biagi (1995a). In this detector, the cathode circles are significantly smaller (diameters in the range of 170-225 μm) and the anode dots ~20 μm. It is manufactured on a Si wafer coated with a 5 μm thick slightly conductive Si oxide layer.

The anodes were connected to the readout lines located on the other side of the silicon wafer (a so called readout bus), as indicates by the “metal 1”in Figure 2. To minimize the electric field distortions produced by the anode bus, floating metalized rings can be added in advanced designs of microdot detectors (Biagi, 1997), see Figure 3. Due to the very small diameters of the anode dots, the detector is cable to operate stably in proportional mode.

Figure 2.

Schematic of a microdot avalanche detector (Biagi, 1995b)

Figure 3.

Schematic of microdot structure. One can see anode dots connected to rows of readout anode lines, cathode circles and floating rings.